Imagine that you’re trying to determine how many candles it would take to boil a cup of water in a pan. To come to an accurate total, you would need to answer several questions. What type of pan is it? Cast iron? Stainless steel? What is the size of the pan? Before being able to accurately predict how many solar panels you need to power your home, several factors need to be taken into consideration.

The first step is to assess your current electrical needs. Your total electricity usage is measured by the kilowatt-hours (kWh) your home uses in a 12-month period. A solar power installer can help you look at past electric bills. The goal is to establish not only how much power your home uses, but what percentage of your electrical needs you would like to be met with solar power.

Some cities, like New York and San Francisco, have online interactive maps to help with estimating the size and cost of solar power systems. Users simply enter their address. Some solar providers utilize specialized software that enables them to give offsite estimates, usually emailed within 24 hours, in response to online requests for a quote.

How Many Solar Panels Do I Need?

Once you have established the number of watts that you would like your solar panels to provide, several other factors must be considered to determine the size of the system, including the amount of sunlight available in your homes location and the orientation of your homes roof to the sun.

Solar modules are categorized by their efficiency, or rated power output, a measurement of the peak power each panel can produce. When estimating how many panels you need to power a home, solar contractors will calculate the roof area needed in square feet.

For example, to generate 2,000 watts from a 12%-efficient system, you need 200 square feet of roof area. The cost of a solar system, including the panels, equipment and installation, is usually described in dollars per watt. While rates vary according to location, a typical system costs $7-10/watt. For the 200-square-feet example used here, the cost of the system would be $14,000 to $20,000.

However, with state and local solar rebates, net metering and federal tax incentives, the cost of installation can be significantly reducedsometimes even cut in half. The federal tax credit program, for example, can cover up to 30% of the cost. In addition, solar contractors now offer financing options, including solar leases with zero down, with monthly payments that are typically less than homeowners were paying on monthly electrical bills.

Today, the average cost for a 10-panel solar power system is $11,000. According to JEA, a community-owned electric utility in Florida that serves 417,000 customers, the average solar panel system saves homeowners 30% on their electric bill. Savings for some customers are much higher, with some home systems producing more electricity than the home uses. That electricity is typically sold back to the grid, providing extra income to homeowners.